Lung cancer causes ~160,000 deaths annually in the U.S. and prevention will be crucial to win the war on this deadliest cancer. Since DNA modification by tobacco carcinogens is one major driver for lung cancer initiation, blocking DNA adduct formation (the root cause) is a plausible strategy. This proposal focuses on the preclinical studies of dihydromethysticin (DHM) as a novel and highly efficacious chemopreventive agent that inhibits lung tumor initiation via preventing tobacco carcinogen-induced DNA modification. Preliminary data demonstrate that DHM (given during carcinogen exposure period at a dose of 50 ppm in diet) completely blocked NNK-induced lung tumor formation in A/J mice. A natural analog, dihydrokavain (DHK), was completely inactive even at 500 ppm in diet. Such a sharp in vivo difference suggests a crucial role of methylenedioxy functional group for specific targeting by DHM. DHM selectively reduced NNAL (the active metabolite of NNK)-induced DNA adducts in the lung tissues. DHM also reduced NNK-derived DNA adducts in F344 rats, indicating its cross-species anti-initiation potential. Based on the reported activity of DHM in activating aryl hydrocarbon receptor (AHR) and our own data of DHM in increasing glucuronidated NNAL in mouse urine, we propose that DHM activates detoxification pathways as the primary mechanism of action. With respect to safety, 17-week dietary exposure of DHM to A/J mice caused no adverse effects at 500 ppm, affording a wide safety margin as a lung cancer chemopreventive agent for long-term use. Our central hypothesis is that DHM effectively prevents tobacco carcinogen-induced lung tumorigenesis, at least in major part, by enhancing carcinogen detoxification potentially via activating AHR leading to a reduction in oncogenic DNA adducts in the target lung tissues. This hypothesis will be tested by accomplishing the following aims: Aim 1 To elucidate the structural determinant(s) of DHM (i.e., intact DHM or its metabolite) for its exceptional in vivo inhibitory activities against NNK-induced DNA adduct formation and lung tumor initiation. Data will also inform the in vivo active form of DHM. Aim 2 To investigate enhanced detoxification as a key mechanism of DHM to inhibit NNK-induced DNA adduct formation and lung tumor initiation. Data will also inform its potential to protect against other tobacco carcinogens and identify surrogate biomarkers for future translational studies. Aim 3 To evaluate the efficacy of DHM against NNK-induced lung tumorigenesis in F344 rats and BaP-induced lung tumorigenesis in A/J mice. Data will inform its cross-species applicability and carcinogen specificity. If the results support our hypothesis, DHM will be well positioned for Good Laboratory Practice (GLP)-based toxicology in higher mammals in preparation for an IND application for human translation studies. The mechanistic knowledge will not only identify surrogate biomarkers critical for translation, but also advance our basic understanding about NNAL metabolism and carcinogenesis.